In the flame spraying process utilized for producing and coating optical structures a flame is directed to the target that is being treated, and a desired material or additive is fed to said flame typically in liquid format. Said material meets the target to be treated typically in particle format, steam or gas. When the material comes into contact with the target to be treated, the properties of the target can be changed. For example the properties, such as colour, of the surface layer of an optical fibre structure, such as a fibre preform or optical fibre, can be changed by means of flame spraying.
Typically a flame spraying apparatus comprises at least one nozzle by means of which a flame is directed to the target to be treated. Conventionally the nozzle comprises several channels that are positioned co-axially. Different components are introduced to the flame through different channels, wherein different material are brought in contact with each other only after the opening of the nozzle. In an embodiment of flame spraying a liquid material is supplied via the innermost channel of the nozzle, and fuel gas, such as hydrogen, is supplied via a ring-like channel surrounding said channel. The flow of hydrogen generates an ejector phenomenon, which, in turn, brings about the absorption of liquid out of the channel, and spraying of said liquid to the gas flow.
The feeding of liquid utilized in flame spraying must be accurate and it must endure back-pressure. In known solutions the pressure of the supplied liquid is generated by means of a pump or it is based on height difference. In piston-type solutions yielding of the feeder as well as leakage resulting therefrom have been detected when high pressures have been used, which have occurred for example when liquid has been pumped through capillaries. Furthermore, as a result of pumping, fluctuation of the pressure level of the liquid often takes place. The throttling of the liquid flow may also cause problems when the possibility of cavitation in the liquid flow increases.
In solutions based on the height difference the aim has been to standardize the pressure by keeping the liquid level at constant height. Therefore in known solutions the location of the container containing liquid is adjusted in the height direction in such a manner that when the liquid content in the container is reduced, the container is moved upwards. Typically, the amount of liquid contained in the container has been estimated on the basis of weight. Such an apparatus is, however, relatively complex in structure, and it is especially difficult to determine the weight of the container containing liquid in real time.